It is commonly known that fluorescent colors provide increased visibility for visual signaling under most lighting conditions, but particularly under low natural lighting conditions. These low natural lighting conditions occur at dusk and also at sunrise and present a challenge for traffic sign manufacturers. If increased visibility of an article is desired, the article is often colored with fluorescent colorants. Fluorescent colors allow enhanced visibility because the visual contrast that the fluorescent colors create with the environment make the materials more conspicuous than ordinary non-fluorescent articles. Fluorescent colored traffic signs are effective at increasing the visibility of the signs which increases motorist safety.
Even though fluorescent signs increase motorist safety, their use for yellow signs has been limited due to the difficulty to obtain a true fluorescent yellow. To date, fluorescent colorants are available in only a limited range of hues. For example, fluorescent colorants are commercially available and include fluorescent red, fluorescent orange and fluorescent yellow-green. However, a true fluorescent yellow which meets the chromaticity requirements of Commission Internationale de l'eclairage (CIE) and ASTM is not readily available. As is known in the art the CIE provides international recommendations for surface colors for visual signaling.
Formulating colors using ordinary or conventional colorants is well known. Ordinary colors do not emit light. Therefore, when formulating colors with ordinary colorants, the important parameters to consider are the light-absorptive and light-reflective properties of the colorants. On the other hand, fluorescent colors do emit light. Therefore, when formulating with fluorescent colorants, the important parameters to consider are the light-absorptive, light-reflective and light-emissive properties of the fluorescent colorants. Due to this distinction between ordinary and fluorescent colors, an added consideration is necessary when formulating colors with fluorescent dyes.
The art of formulating colors from ordinary colorants is well-developed. For example, it is known that a mixture of blue colorant with a red colorant will give a purple color. However, the art of formulating colors from fluorescent colorants is not well-defined. U.S. Pat. No. 4,443,226 issued to Rohser describes combining thioindigo and/or derivatives of the red and pink series of thioindigo with specific yellow disperse dyestuffs to obtain a shade of fluorescent orange-red as required to meet color point, luminance and fastness to light.
The need exists for fluorescent yellow articles such as those useful for visual signaling, for example, traffic signing. The art does not currently possess such fluorescent yellow articles nor an obvious way to achieve them.